Different designs of fuel injection systems with an electromagnetic control of the needle valve of the fuel injector are known and disclosed for example in the following publications: Swiss patent specification No. 434,875; U.S. Pat. Nos. 3,464,627 and 3,610,529; German published patent application No. 3,227,742 (and the corresponding U.S. Pat. No. 4,566,416) as well as European published patent application No. 0 228 578 (and the corresponding U.S. patent application Ser. No. 06/938,396 filed on Dec. 1, 1986). The injectors disclosed in the above mentioned publications are so-called accumulator fuel injectors. It is a characteristic of such accumulator injectors that an accumulator volume or chamber, having a volume substantially larger than the maximum volume of the fuel injected during each injection event, is provided in the injector body upstream of the seat of the injector needle valve. The injection orifices are located downstream of this seat. These orifices communicate with the combustion chamber of the related internal combustion engine. The fuel stored in the accumulator chamber under high pressure is partly discharged through the injection orifices during each injection event with a simultaneous pressure drop in the accumulator chamber. The injector's accumulator chamber communicates with the high pressure fuel supply line of the injection system via a restriction or orifice. The orifice, due to its small flow passage, prevents the formation of noticeable pressure waves in the fuel supply lines during each injection event. Such pressure waves would highly affect the uniform fuel distribution in a multicylinder engine and the stability of the injection events of a single injector from cycle to cycle.
To obtain good engine performance with respect to engine power, efficiency and emissions, a very uniform fuel distribution from cylinder to cylinder must be achieved in a multicylinder engine at each engine operating point. The same holds true for each injector from cycle to cycle. In addition to said orifice between each injector accumulator chamber and the fuel supply line, a plenum fuel chamber communicating with the fuel supply lines of all injectors of the injection system is provided in the above mentioned prior art injection systems in order to obtain a uniform fuel distribution. Due to its relatively large volume, the plenum chamber evens out the pressure pulsations created by the high pressure fuel supply pump and thus creates a constant pressure level for all injectors. However, at different points in the engine operation range different fuel injection pressures are required. As an example, it is advantageous to use a low injection pressure at low engine load and at idling and a high injection pressure at high engine load and speed.
In a passenger car engine, transient speed and load conditions are the most often encountered situations, and the injection system pressure must be rapidly adapted to the varying demand of the driver. It must be possible to rapidly increase and drop the fuel pressure in the plenum chamber. Due to its large volume, this is a difficult task to accomplish.
It is now an object of the present invention to provide a new and improved fuel injector which allows to eliminate the aforementioned plenum chamber while nevertheless maintaining the required uniform fuel distribution at each engine operating point.
This and other objects are accomplished by a fuel injector as claimed in claim 1.
Because of the particular design of the injector needle valve piston of the injector it is also possible to place the accumulator chamber which is normally arranged within the injector body outside of the latter at any suitable location within the high pressure part of the fuel injection system. Said orifice between the injector accumulator chamber and the fuel supply line is eliminated too. Because of the novel design of the injector needle valve piston, it is no longer important that the pressure level in the fuel supply line to each injector remains constant. Pressure pulsations are thus allowed, as long as they are the same for all injectors.
Those and further advantages of the invention become apparent from the following detailed description of various embodiments shown in the drawings.